This invention relates to methods and systems for generating a broadband spectral continuum and continuous wave-generating systems utilizing same.
Trends in DWDM sources include: increasing channel counts; narrowing channel spacing; and the use of an array of single-frequency laser diodes. However, the use of such arrays of laser diodes requires high cost for separate wavelength stabilization; inventory problems in case of channel failure; and large space required for installation of such arrays.
A spectrum-slicing scheme has been researched as a broadband wavelength division multiplexing (WDM) source. The broad spectrum for slicing is obtained by various approaches including amplified spontaneous emission (ASE), a light-emitting diode, spectral broadening due to self-phase modulation, a short-pulse mode-locked laser (MLL), and SC generation. Recently, the use of the longitudinal modes of an MLL pulse train as CW channels has been discussed in spectrum-slicing multiple wavelength sources. The technical advantage of the scheme is its precise spectral mode separation solely determined by the MLL pulse repetition rate. To generate the longitudinal modes, an MLL or an amplitude-modulated distributed feedback laser is used.
However, there are various problems related to the spectrum-slicing concept. The schemes utilizing the nonlinear spectral broadening of an optically amplified pulse suffers from amplitude jitter due to four-wave mixing of ASE. In longitudinal mode slicing schemes, the narrow spectral extent (xcx9c15 nm) and non-uniform power distribution among modes hinder their application as broadband WDM sources. In addition, very high modulation- or repetition-rates are required to satisfy current channel spacing standards.
U.S. Pat. No. 5,631,758 discloses a chirped-pulse multiple wavelength telecommunications scheme. Channel spacing is set by a modulator.
The Article by J. J. Veselka et al., entitled xe2x80x9cA Multi Wavelength Source Having Precise Channel Spacing for WDM Systemsxe2x80x9d, IEEE Photon., Technology Lett., Vol. 10, pp. 958-960, 1998 discloses an amplitude-modulated CW laser wherein channel spacing is set by the repetition rate of the amplitude modulation.
The article by H. Sanjoh et al., entitled xe2x80x9cMultiwavelength Light Source With Precise Frequency Spacing Using A Mode-Locked Semiconductor Laser And An Arrayed Waveguide Grating Filterxe2x80x9d, IEEE Photon., Technology Lett., Vol. 9, pp. 818-820, 1997 discloses a mode-locked laser with channel spacing actively determined by pulse repetition rate of the laser and demultiplexer settings. Because the spectral range for CW channels is not flat, external flattening is required.
An object of the present invention is to provide a method and system for generating a broadband spectral continuum and a continuous wave-generating system utilizing same.
Another object of the present invention is to provide a method and system for generating a broadband spectral continuum and a continuous wave-generating system utilizing same wherein stabilization is simplified and the generation of multi-wavelengths is cost-effective.
In carrying out the above objects and other objects of the present invention, a method for generating a broadband spectral continuum is disclosed. The method includes the steps of generating soliton pulses at a repetition rate greater than or equal to 1 Gb/s. The method further includes compressing the higher-order soliton pulses in a temporal domain through soliton-effect compression to obtain compressed soliton pulses having a spectrum. The method finally includes breaking up the compressed soliton pulses to shape the spectrum of the compressed soliton pulses through higher order dispersion effects and self-phase modulation to obtain the broadband spectral continuum. The continuum has a plurality of substantially uniform longitudinal modes each of which is a narrow band continuous wave in the time domain. The modes are spectrally spaced based on the repetition rate.
Preferably, the step of compressing includes the step of launching the higher-order soliton pulse into a first end of an anomalous dispersion fiber including at least one pulse compression stage having a length based on the order of the soliton pulse and a spectral shaping stage. The step of breaking up is performed adjacent a second end of the anomalous dispersion fiber in the spectral shaping stage. The spectral shaping stage is typically a dispersion-shifted optical fiber section of the anomalous dispersion fiber.
The step of compressing may be performed in multiple pulse compression stages of the anomalous dispersion fiber.
The higher order dispersion effects typically include third order dispersion effects. The shape of the broadband spectral continuum is based primarily on the sign and magnitude of third order dispersion of the spectral shaping stage adjacent the second end. The shape of the broadband spectral continuum is also based on pulse width of the compressed soliton pulse immediately prior to the step of breaking up. The dispersion effects typically include second and third order dispersion effects, and wherein the magnitude of the second order dispersion and the magnitude of the third order dispersion normalized by the pulse width have substantially the same order of magnitude adjacent the second end of the anomalous dispersion fiber in the spectral shaping stage.
The spectral continuum is typically greater than 15 nm wide.
Further in carrying out the above objects and other objects of the invention, a system is provided for generating broadband spectral continuum. The system includes a soliton pulse generator including a single laser for generating soliton pulses at a repetition rate greater than or equal to 1 Gb/s. The system also includes an optical fiber including at least one pulse compression stage for compressing the soliton pulses through soliton-effect compression to obtain compressed soliton pulses having a spectrum. The optical fiber further includes a spectral shaping stage optically coupled to the at least one pulse compression stage for breaking up the compressed soliton pulses adjacent an output end of the fiber to shape the spectrum of the compressed soliton pulses through higher order dispersion effects and self-phase modulation to obtain the broadband spectral continuum. The continuum has a plurality of substantially uniform longitudinal mode each of which is a narrow band continuous wave in the time domain. The modes are spectrally spaced based on the repetition rate.
The at least one pulse compression stage of anomalous dispersion fiber may have a dispersion which is relatively constant therein.
The anomalous dispersion fiber may be a second pulse compression stage less than 100 and even less than 30 meters in length.
The dispersion effects typically include third order dispersions having a sign and a magnitude. The third order dispersions may have either a positive sign or a negative sign.
The system may include a plurality of pulse compression stages of anomalous dispersion fiber for compressing the soliton pulses through soliton-effect compression to obtain the compressed soliton pulses.
The length of the at least one pulse compression stage of anomalous dispersion fiber is typically based on a minimal width of the compressed soliton pulses.
Yet still further in carrying out the above objects and other objects of the invention, a system for generating narrow band, continuous waves (CW) substantially simultaneously on multiple CW channels at multiple wavelengths and with channel spacing of at least 1 GHz is provided. The system includes a soliton pulse generator including a single laser for generating soliton pulses at a repetition rate greater than or equal to 1 Gb/s. The system also includes an optical fiber having at least one pulse compression stage and a spectral shaping stage. The at least one pulse compression stage receives the soliton pulses at a first end of the fiber and the spectral shaping stage generates a broadband spectral continuum within the fiber and provides the broadband spectral continuum at a second end of the fiber. The continuum has a plurality of substantially uniform longitudinal modes each of which is a narrow band continuous wave in the time domain. The modes are spectrally spaced based on the repetition rate. The system also includes at least one optical filter coupled to the second end of the fiber for spectrally slicing the longitudinal modes from the broadband spectral continuum to obtain the continuous waves.
The system may include a time domain multiplexer to adjust spacing between the longitudinal modes. The multiplexer may include a pulse interleaver to increase channel spacing.
The single laser may be a single mode-locked laser such as a mode-locked erbium-doped fiber laser or a mode-locked semiconductor laser.
The single laser may be a single continuous-wave (CW) laser adapted to be externally modulated.
The at least one optical filter may include a monochromator, an arrayed wave guide grating, a Fabry-Perot, a Mach-Zehnder, or fused-tapered couplers.
The length of the spectral shaping stage may be less than 10 meters.
The spectral shaping stage may be a dispersion-shifted fiber.
The optical fiber may include a polarization preserving fiber.
The spectral continuum is typically greater than 15 nm wide.
The soliton pulse generator may include a polarization controller for aligning polarization of the soliton pulses with a polarization eigenmode of the at least one stage.
The system may include a fiber amplifier or a semiconductor optical amplifier for amplifying the soliton pulses. The fiber amplifier may be an erbium-doped fiber amplifier.
The optical fiber may include a high-nonlinearity (Hi-NL) fiber, a polarization preserving fiber, or a dispersion decreasing fiber.
The problems of narrow, non-uniform spectral extent are overcome with the fiber-based SC generation and the amplitude jitter is overcome by modifying the SC generation and a slicing scheme. As disclosed in the above-noted application, SC in optical fiber results from the interaction between self-phase modulation and third-order dispersion and forms a spectral region with xc2x10.5 dB flatness over  greater than 30 nm. Based on this, one can obtain substantially uniform longitudinal modes over a broad spectral range by seeding the SC with a high repetition rate pulse source. For example, a 140 nm SC at 10 GHz repetition rate can be obtained with an MLL and a dispersion shifted (DS) fiber.
To suppress the adverse effect of amplitude jitter on the transmission performance, the SC generation and slicing processes can be modified. First, the reduction of amplitude jitter due to noise-seeded four-wave mixing is considered herein. It is previously reported that during the SC generation in DS fiber, the ASE mapping through four-wave mixing leads to spectral coherence degradation increasing with propagation distance. The random nature of ASE causes fluctuations in amplitude and frequency.
To reduce the resultant deterioration in signal-to-noise ratio, the length of DS fiber is minimized and the reduction in spectral broadening is compensated by increasing the amplification in the present invention.
The influence of other longitudinal modes is also considered herein. To suppress the interference from adjacent modes that can be converted into amplitude jitter in the spectrum-slicing process, the pulse repetition rate is increased and, consequently, the mode spacing. With a time-domain multiplexing method, it is possible to increase them without changing the source repetition rate.
After the time-domain multiplexing, each longitudinal mode is sliced as a CW channel. To check the degradation in CW amplitude stability, one can measure RIN values at various wavelengths and compare them with source laser RIN value. The RIN degradation is restrained within 7 dB/Hz in both anti-Stokes and Stokes regions as described herein. The CW channels near the source wavelength experience higher levels of degradation.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.